This invention relates generally to video circuits for use in video display terminals and particularly to such circuits that enable selective control of the brightness of one or more areas of the display.
Video display systems are well known in the art. They generally comprise a cathode ray tube (CRT) driven by a character generator under the control of a microprocessor control means for selecting the signals to be produced by the character generator for display. The character generator output consists of a series of pulses that are supplied to a video amplifier. The video amplifier processes the information and supplies it to the CRT for creating a video display, all in a well known manner. The video display terminal usually has a user operated keyboard coupled to the microprocessor control means and may include the facility for accepting a computer input for performing specialized tasks such as wordprocessing.
Many video display terminals also include means for displaying one or more pixels or sections of data in different levels of contrast. Some also have the capability of displaying sections in a much greater brightness of highlighting. The highlighting may be under direct control of the user through the keyboard or may be software-based for automatically displaying certain portions of the image in very high brightness. In prior art systems including both these features, namely different contrast levels (gray scale) and selective high brightness control for portions of the display, the gray scale feature is generally overridden or lost in the portion of the display that is highlighted. This is not generally desirable. With the present invention, a simple circuit arrangement is provided for retaining the multilevel gray scale feature even in highlighted areas.